Piston rings used in internal combustion engines have been increasingly used under harsh high-temperature, high-pressure environments in recent years, demanding further improvement in wear resistance, running-in property, low friction, and the like. In response to such demands, Patent Document 1 (Japanese Laid-Open Patent Application No. 2012-202522), for example, proposes a piston ring provided with a carbon-based coating exhibiting low friction and wear resistance. Specifically, Patent Document 1 proposes a piston ring having a laminated coating comprising two types of layers having different hardnesses laminated in at least two layers, a hardness difference between the two types of layers being 500 to 1700 HV, a high hardness layer having the same or greater thickness than a low hardness layer, and the coating as a whole having a thickness of 5.0 μm or greater. At this time, the low hardness layer is formed by sputtering, and the high hardness layer is formed by ion plating.
Further, Patent Document 2 (Japanese Laid-Open Patent Application No. 2007-169698) proposes a piston ring having an amorphous hard carbon coating excellent in adhesiveness to a piston ring base material, high in hardness, and excellent in wear resistance. Specifically, Patent Document 2 proposes a piston ring having a first amorphous hard carbon layer formed on a surface of the piston ring base material and substantially consisting of only carbon without hardly any hydrogen, and a second amorphous hard carbon layer formed on a surface of the first amorphous hard carbon layer and substantially consisting of only carbon. When viewed from a cross section, a transmission electron microscopic image of the first amorphous hard carbon layer is brighter than that of the second amorphous hard carbon layer.